Specimen Current (The Specimen As Detector)

Specimen Current (The Specimen As Detector)

Consider the interactions of the beam electrons with the specimen to produce backscattered and secondary electrons. For a 20 keV beam on copper, about 30 out of 100 beam electrons are backscattered ( = 0.3). The remaining 70 beam electrons lose all their energy in the solid, are reduced to thermal energies, and are captured by the solid. About 10 units of charge are ejected as secondary electrons ( = 0.1). This leaves 60 excess charges in the target. What is the fate of these electrons? To understand this, an alternative view is to consider the currents, defined as charge per unit time, which flow in and out of the specimen. Viewed in this fashion, the specimen can be treated as an electrical junction, as illustrated schematically in Figure 4sc.1, and is subject to the fundamental rules which govern junctions in circuits. By Thevinin's junction theorem, the currents flowing in and out of the junction must exactly balance, or else there will be net accumulation or loss of electrical charge, and the specimen will charge on a macroscopic scale. If the specimen is a conductor or semiconductor and if there is a path to ground from the specimen, then electrical neutrality will be maintained by the flow of a current, designated the "specimen current" (also known as "target current" or "absorbed current"), either to or from ground, depending on the exact conditions of beam energy and specimen composition. What is the magnitude of the specimen current?

Considering the specimen as a junction, the current flowing into the junction is the beam current, iB, and the currents flowing out of the junction are the backscattered electron current, iBSE, and the secondary electron current, iSE. For charge balance to occur, the specimen current, iSC, is given by:

iSC = iB - iBSE - iSE (4sc.1)

For the copper target, the backscatter current will be iBSE =  iB = 0.3 iB and the secondary electron current will be iSE =  iB = 0.1 iB. Substituting these values in equation (4sc.1) gives the result that the specimen current will be iSC = 0.6 iB, double the largest of the conventional imaging currents, which is the backscattered electron signal. If a path to ground (literally a wire from the specimen to a grounding point on the SEM stage) is not provided so that the specimen current can flow, the specimen will rapidly charge.

Note that in formulating equation (4sc.1) no consideration is given to the large difference in energy carried by the backscattered and secondary electrons. Since current is the passage of charge per unit time, the ejection of a 1 eV secondary electron from the specimen carries the same weight as a 10 keV backscattered electron.

The specimen obviously serves as its own collector for the specimen current. As such, the specimen current signal is readily available just by insulating the specimen from ground and attaching a wire to ground to collect the specimen current. Does the specimen current signal actually convey useful information? As described below under contrast formation, the specimen current signal contains exactly the same information as that carried by the backscattered and secondary electron currents. Since our detectors measure a convolution of backscattered and/or secondary current with other characteristics such as energy and/or directionality, the specimen current signal can give a unique view of the specimen (Newbury, 1976).

To make use of the specimen current signal, the current must be routed through an amplifier on its way to ground. The difficulty is that we must be able to work with a current similar in magnitude to the beam current, without any high gain physical amplification process such as electron-hole pair production in a solid state detector or the electron cascade in an electron multiplier. To achieve acceptable bandwidth at the high gains necessary, most current amplifiers take the form of a low input impedance operational amplifier (Fiori et al., 1974). Such amplifiers can operate with currents as low as 10 pA and still provide adequate bandwidth to view acceptable images at slow visual scan rates (1 500-line frame/s).

Compositional Contrast with Specimen Current

To understand the appearance of the specimen in an image prepared with the specimen current signal, consider how the signals change between any two different locations. Using equation (4sc.1) as a starting point, the difference in signals between any two locations can be calculated. To simplify the argument, consider that the backscattered and secondary electron signals are combined into an "emissive" signal:

iE = iBSE+ iSE (4sc.2)

With this substitution, equation (4sc.1) becomes:

iB = iE + iSC (4sc.3)

The difference in the signals between any two pixels is found by taking differences for each term:

iB = iE + iSC (4sc.4)

Because the electron optical column is carefully constructed to maintain a constant beam current, the difference in the beam current between any two points in the image is zero, except for statistical fluctuations. Equation (4sc.4) can thus be rearranged to give the relationship between the emissive and the specimen current signals:

iB = 0

iSC = -iE (4sc.5)

The sense of the contrast is thus opposite in the specimen current image compared to the image recorded with a detector of emissive mode signals.

Images of Composition in the Specimen Current Signal

The contrast reversal predicted by equation 4sc.5 can be seen in Figure 4sc.2, which is an image of a flat specimen with regions of differing composition. Where certain regions appear dark in the emissive mode image of Figure 4sc.2(a), these same images appear bright in the specimen current image of Figure 4sc.2(b).

While this contrast reversal may seem to be a trivial change, a more subtle difference exists between the specimen current and emissive mode signals. Specimen current is sensitive only to the numbers of electron electrons leaving the specimen and is completely insensitive to their trajectories. As long as a secondary or backscattered electron leaves the specimen, it contributes information to the specimen current signal, regardless of its fate to be collected or to be lost. Of all possible detectors, specimen current is the only detector which is sensitive to number effects only. Trajectory and energy effects are completely eliminated. If the specimen is biased to suppress secondary emission, the specimen current signal can be rendered sensitive to backscattered electron effects only (Heinrich 1966).

Because the direct specimen current image gives the reversed sense of atomic number contrast, it is common practice to apply a signal processing operation to artificially reverse the sense of the signal (reversed specimen current image) so that a brighter area corresponds to higher atomic number.

Images of Topography in the Specimen Current Signal

To understand the appearance of the topography specimen current image, two properties of specimen current must be recalled. First, from equation (4sc.5), the sense of topographic contrast is expected to be reversed in the specimen current image as compared to the emissive mode image. Second, specimen current is only sensitive to number effects, and is completely insensitive to trajectory effects, which contribute strongly to emissive mode images prepared with the E-T detector, and effects of the energy distribution of backscattered electrons, to which solid state detectors are sensitive.

The direct specimen current image of a rough, fractured surface of high purity iron is shown in Figure 4sc.3(a). The sense of the topography appears reversed relative to the emissive mode image, Figure 4sc.3(b), as expected from equation (4sc.5). The fine scale dimples on the grain surfaces appear prominently in the specimen current image. An unexpected effect is the uniform appearance of facets at approximately equal tilt to the beam. The facets of the grain boundary triple junction in the lower left of the image appear uniform in the specimen current image, but these facets have different brightness in the positively-biased E-T image and in the dedicated backscattered electron image because of trajectory effects, which are completely absent in the specimen current image. Since both backscattering and secondary electron emission increase monotonically with tilt, the magnitude of the specimen current can be used to quantitatively assess the local tilt angle.

The distraction of the reversal of topography encountered in the direct specimen current image can be eliminated by using contrast reversal in the signal processing chain to produce the image shown in figure 4sc.3(c), which displays the proper sense of the topography, as expected from the point of view of the emissive mode detector. Unless this contrast reversal is applied, the sense of topography obtained from a direct specimen current image will be incorrect.

Signal processing for specimen current

Situations sometimes arise in which it is desirable to reverse the contrast which naturally appears in an image. An example is the direct specimen current image, which has the opposite sense of contrast to the corresponding emissive mode image. Since we intuitively expect to interpret images according to the characteristics of the emissive mode, direct specimen current images are often confusing, both for topographic contrast, where the sense of topography appears reversed, and for atomic number contrast, where light elements are unexpectedly bright compared to heavy elements. It is therefore useful to artificially reverse the contrast during signal processing. This reversal is accomplished by the following signal transformation:

Sout = Smax - Sin(4sc.6)

An example of this contrast reversal transformation applied to an image of a rough surface is shown in Figure 4sc.3(c).

Specimen Current for Separation of Contrast Components

The specimen current image is invaluable as a reference image for separation of contrast components through comparison of emissive (backscattered and secondary electron) images with specimen current images (Newbury, 1976). The specimen current signal is totally dominated by number effects and is completely independent of trajectory effects, while an asymmetric backscattered electron detector such as the negatively-biased E-T detector is extremely sensitive to trajectory effects. Figure 4sc.4 shows a comparison of images of a two phase lead-tin eutectic alloy with surface topography. Figure 4sc.4(a)shows a negatively-biased E-T detector and Figure 4sc.4(b) the corresponding specimen current image, with the contrast reversed so that the phase with the higher average atomic number appears bright. In Figure 4sc.4(a), the ridges of the topography dominate the image because the negatively-biased E-T detector is located at a low take-off angle above the surface, which increases the effect of the apparent oblique illumination, and increases the sensitivity of the image to the trajectory effects inherent in the topographic contrast. In Figure 4sc.4(b), the atomic number contrast dominates the image obtained with the specimen current signal, and the topographic contrast is greatly diminished.

Because the specimen current image is only sensitive to the numbers of SE and BSE leaving the specimen and not to their trajectories, all of the shadowing effects encountered in conventional emissive mode images (BSE only or SE+BSE)are eliminated from specimen current image.

References

Fiori, C. E., Yakowitz, H., and Newbury, D. E. (1974). SEM/1974, IIT Research Institute, Chicago, Illinois, p. 167.

Heinrich, K. F. J. (1966). In Proc. 4th Intl. Conf. on X-ray Optics and Microanalysis, eds. R. Castaing, P. Deschamps, and J. Philibert, Hermann, Paris, 159.

Newbury, D. E. (1976). SEM/1976/I, IIT Research Inst., Chicago, Illinois, 111.

Figure Captions

4sc.1 Illustration of currents which flow in and out of specimen: iB , beam current; iBS, backscattered electron current; iSE, secondary electron current; iSC, specimen current. The junction equivalent of the specimen is also shown.

4sc.2 Atomic number (compositional contrast) observed in a composite specimen consisting of a copper grid, silicon chip (square, and silver dag on an aluminum stub. (a) Backscattered electron image derived from a negatively-biased Everhart - Thornley detector. (b) Direct specimen current image of the same region; note the contrast reversal compared to 4sc.2(a)and the complete lack of the strong shadowing seen in the highly directional BSE image.

4sc.3(a) Fracture surface of polycrystalline iron viewed with various detectors: (a)Negatively-biased Everhart-Thornley, BSE only; (b) Positively-biased Everhart-Thornley, SE+BSE; (c) Dedicated backscattered electron detector having a large solid angle (sum mode)and placed at a high take-off angle, showing compositonal differences. (d) Dedicated backscattered electron detector having a large solid angle (difference mode)and placed at a high take-off angle, showing topography; (e) Direct specimen current signal showing contrast reversal from (a)(b), and (c); (f) Reversed contrast specimen current image showing same general sense of contrast as (a). E0 = 15 keV.

4sc.4 Separation of contrast components (composition and topography) in a lead-tin eutectic alloy with (a) highly directional, low take-off angle, negatively-biased Everhart - Thornley detector collecting only BSEs; weak atomic number contrast is observed, but the strongest features are surface irregularities (b)specimen current (contrast reversed, where the atomic number contrast dominates and the topography is almost completely eliminated.